Magnetic labels facilitate the separation and purification of chemical and biological samples. These labels are often superparamagnetic nano- or microspheres (e.g., Invitrogen's Dynabeads™), which can be covalently attached to most chemical and biological samples. For example, immunomagnetic separation employs antibodies—often monoclonal—bound to magnetic particles for the removal of prokaryotic and eukaryotic cells from suspension. Many techniques—including cell separation, free flow magnetophoresis, and immunoassays—have been developed for use in microfluidic devices for lab-on-a-chip technology. These magnetic labeling techniques have several limitations: i) the separations are binary: magnetic particles are separated from diamagnetic particles; ii) the labeling of a diamagnetic material requires a chemical reaction; iii) the presence of a magnetic particle attached to a diamagnetic material—specifically a cell, antibody, or protein—alters the functionality and properties of the surface of the material; and iv) the magnetic label must be removed after separation to obtain a pure diamagnetic sample.
In the last decade, the magnetic levitation of diamagnetic materials has become more accessible to standard laboratory facilities as the early experimental setup consisting of superconducting magnets (>10 T) and a pressurized oxygen atmosphere has been replaced by small rare-earth magnets and aqueous paramagnetic salt solutions. One of the characteristics of magnetic levitation is that there is only one position in a magnetic field in which an object is stably levitated. When a levitating object in magnetic fields is moved away from a position of equilibrium, a restoration force on the object returns it to equilibrium position. This stable point is determined by its volume magnetic susceptibility and density. Therefore, different substances levitated in the same magnetic field have different equilibrium positions of levitation and can thus be separated.
As an extension of levitation, diamagnetic traps have been developed to control, manipulate, and direct the positioning of cells and polymer microspheres suspended in solution. Magnetic field maxima can exist only at the source of the field and therefore stable trapping of materials having greater magnetic susceptibility than their environments occurs at the source of the field. Magnetic field minima can be achieved outside or spaced a distance from a magnetic field source. The magnetic minima have been used to levitate and confine biological materials and other diamagnetic materials. For example, in microfluidic systems, diamagnetic particles suspended in ferrofluid or an aqueous paramagnetic solution have been trapped and their trajectory manipulated while traversing the inhomogeneous magnetic fields.
Magnetic separations are used extensively in biomedicine, and other areas, usually in systems that separate magnetic particles (or magnetically-labeled particles) from diamagnetic media. Magnetic media has also been used to separate diamagnetic particles, with ferrofluids providing the largest magnetic response.
A variety of sensors are available for measuring densities of liquids and solids.
For liquids, floating bulb hydrometers estimate density values with accuracies of ±0.01 g/cm3. This method is simple, portable, and does not require electricity for measuring densities of large volumes of fluids (>10 mL). Pycnometers are more accurate (ρ=±0.001-0.0001 g/cm3), but lack portability. Pycnometers require accurate measurements of both mass and volume of the sample and, therefore, are highly dependent on the availability and accuracy of an analytical balance. It also requires milliliter volumes of fluid to obtain an accurate measurement of volume. Density measurements with accuracies of ±0.00001 g/cm3 can be obtained using oscillating-tube density meters. This technology uses a resonating glass or metal tube of fixed volume; the density of the liquid filling the tube can be determined through its relationship to the resonant frequency of the tube. These instruments cost several thousand dollars, but offer the desirable characteristics of portability (by using batteries as the source of energy), automation, high-throughput, and the ability to process volumes of 1-5 mL. Recently, Sparks and co-workers have developed a lab-on-a-chip version of this device that is capable of measuring density values of liquids using volumes as small as 0.5 μL.
Hydrostatic weighing is a common technique for measuring densities of solids. It relies on the Archimedes principle and requires an accurate measurement of both the mass of the solid and the volume of the liquid that it displaces on the hydrostatic balance. This technique is useful for relatively large solids that produce detectable changes in volume upon submersion. Pycnometers also can be used for measuring densities of solids. The weight of the solid is obtained using an analytical balance and the volume is carefully measured by the amount of liquid that the solid displaces within the pycnometer. Density-gradient columns are a standard and accurate method for measuring densities of solids—usually plastics—with non-uniform shapes and with sensitivities of ±0.0001 g/cm3. This method operates on observations of the level to which a sample sinks in a column of liquid containing a density gradient. The mass and volume of the sample does not need to be measured accurately. The method, however, is time consuming (several hours per measurement) and requires the use of expensive standards with known densities.